The invention is directed to optical waveguide fiber lasers configured to share a common source of pump light. The novel fiber laser configuration includes modular units for laser wavelength selection.
High performance telecommunication systems may be made more cost efficient by using wavelength division multiplexing (WDM). Usually, modifications are needed only at the receive and transmit ends of the system and perhaps at already existing regenerator stations. The waveguide fiber capability is more fully utilized and very high transmission rates are made possible without incurring the cost of installing additional waveguide fiber cables.
A major part of the cost of a fiber laser sources for use in WDM systems is the pump laser. The pump laser itself is expensive and the utilization of the pump power is usually less than optimum.
There is therefore a need in the telecommunications industry for a strategy to reduce pump laser cost by more fully utilizing the pump laser power. However, this more efficient use of pump laser power must not reduce the design flexibility of WDM systems. Thus a successful strategy for low cost system upgrade must include to some degree the notion of interchangeable parts, for example interchangeable and compatible modules which perform a specific communication system task.
The present invention meets the need for lower cost multiple wavelength laser sources and provides the additional flexibility of modular design.
A first aspect of the invention is an optical circuit made up of a plurality of optical waveguide fiber lasers which are coupled to a common pump laser by means of a 1xc3x97N optical coupler. The waveguide fiber is made a laser cavity by a reflective element at one end of the fiber and a means for wavelength selective feedback at the opposite end. To make the multiple wavelength laser source suitable for wavelength division multiplexing systems, a particular wavelength selective feedback means selects a wavelength different from that selected by each of the other wavelength selective feedback means. However, it is clear that one or more laser pairs may produce the same light wavelength if a particular system requires such a configuration.
Wavelength selective feedback means are devices which select light of a particular wavelength or wavelength band and direct the light back into the laser fiber. The selection means may be one in which a particular wavelength or wavelength band is reflected or transmitted. Devices which perform this function include a grating, a loop mirror, a bandpass filter, an etalon filter, an interferometric filter, a dielectric or metallic mirror, and combinations of these devices. The mirrors may be plane or curved.
A particularly useful feature of the novel optical circuit is the modular design of the circuit components. In this document modular means the optical device (module) has at least one input port for receiving light and at least one output port for transmitting light, except for source or detector modules which may have only one output or input port respectively. The transmitting and receiving ports of different modules are compatible in that the input port of any module is equipped with optical connectors or couplers which can efficiently receive light from the output port of any other module. A large number of effective designs of optical connectors and couplers are available in the telecommunication art.
A further aspect of the novel optical circuit includes an external modulator optically coupled to a fiber laser to encode information into the laser light. Various embodiments of the optical circuit, including an external modulator, may be constructed by coupling of optical amplifiers into the circuit, either before or after the external modulator. The optical amplifiers are designed so that the same pump laser which supplies power to the fiber lasers may be coupled to supply pump light to the amplifiers as well.
The pump lasers are an expensive component in optical circuits which employ lasers or amplifiers. The circuit design in which light is distributed from a common pump laser to several optical fiber lasers or to several optical amplifiers makes most efficient use of the pump laser and thus significantly reduces circuit cost.
In addition, the concept of modular optical component design greatly enhances flexibility of the optical circuit. For example, the pump laser, the fiber lasers, the 1xc3x97N couplers, the modulators, or amplifiers, may each be made modular to provide maximum flexibility. As an alternative, groups of components such as a fiber laser, a wavelength selective feedback device and an amplifier may be made into a module for interface with another module, such as an external modulator, which provides for a particular circuit function. Further examples of alternative modular designs are presented below.
The silica based optical waveguide fiber has a window of low attenuation and can be made to have a coincident window of low dispersion over the wavelength range of about 1520 nm to 1575 nm. This operating window coincides with that of erbium doped fiber lasers and amplifiers and is therefore the preferred operating window for the novel optical circuits disclosed herein.
The devices for selective feedback of light are capable of discriminating between wavelengths separated by about 0.1 nm.
A further aspect of the invention is the use of the multiple fiber lasers, having a common pump source, together with external modulators to form a transmitter for use in a wavelength division multiplexed system. The width of the operating window and the capabilities of the wavelength selective feedback devices provide the opportunity for multiplexing a large number of signals onto a single waveguide fiber.